Abstrak 
Molecular Weight Effects in the Third-Harmonic Generation Spectroscopy of Thin Films of the Conjugated Polymer MEH-PPV
Ayi Bahtiar, Kaloian Koynov, Taek Ahn, Christoph Bubeck
Unpad
Inggris
Unpad
aggregation, chain orientation, conjugation length, local field, Molecular weight, refractive index, Sellmeier coefficients, third-harmonic generation, three-photon resonance
Thin spin-cast films of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEHPPV) were prepared from samples whose weight-average molecular weight (Mw) was varied in the range of 10-1600 kg/mol. We have characterized the films by means of transmission and reflection UV-Vis-NIR spectroscopy to derive the linear optical constants, and third-harmonic generation spectroscopy with variable laser wavelengths to get the modulus and phase angles of the complex third-order nonlinear optical susceptibility ????. Increasing molecular weight yields films with significantly larger ?(3) values, absorption coefficients and refractive indices. The ?(3) values of films from the largest and lowest Mw differ by a factor of 4 which is caused by chain orientation effects, local field effects, and changes of the effective conjugation length.